Compact and slim electronic devices include many flexible printed wiring boards. To achieve higher-density and thinner electronic devices, the flexible printed wiring boards having a multilayer structure have been more required. In addition, as the flexible printed wiring boards are required to be higher quality and more inexpensive, it is difficult to fulfill such high requirements for the flexible printed wiring boards.
In order to make a flexible printed wiring board with a multilayer structure, it is necessary to stack, on a core material including an electrically insulating layer (first electrically insulating layer) composed of a polyimide film or the like and a conductive layer (first conductive layer), another electrically insulating layer (second electrically insulating layer) and another conductive layer (second conductive layer). The technique for forming the second electrically insulating layer and the second conductive layer is exemplified by a method of using a resin sheet with a metal foil, in which the resin sheet includes a metal foil and an adhesive resin layer in a B-stage state. The adhesive resin layer is made from, for example, an epoxy resin composition having excellent adhesion properties and electrical insulation properties.
However, when such a resin sheet with a metal foil is used, the electrically insulating layer composed of a cured product of the resin layer has insufficient easy-bending properties. On this account, such a resin sheet has not been able to satisfy recent high requirements for flexible printed wiring boards.
In order to improve the easy-bending properties of the cured product of a resin layer of a resin sheet with a metal foil, there has been proposed a method of forming an anchor coat layer containing a polyimide on a metal foil and forming an adhesive resin layer on the anchor coat layer. The anchor coat layer containing a polyimide is formed by applying a varnish containing a polyamic acid that is a precursor of the polyimide onto a metal foil layer and heating the varnish to form a film.
In order to form the anchor coat layer containing the polyimide, it is necessary to cause imidization of the polyamic acid by promoting cyclization of the polyamic acid by heating the varnish, and this requires heating of the varnish at a high temperature of 300° C. or higher. When heated to such a high temperature, the anchor coat layer would greatly contract as a result of cooling, and such contraction may cause a warp of the resin sheet with a metal foil. A largely warped resin sheet with a metal foil may impair processing stability when the resin sheet with a metal foil is used or may impair the dimensional stability of a product produced by using the resin sheet with a metal foil.
To address this problem, the present applicant has developed a varnish for forming an anchor coat layer, and the varnish contains a polyimide having an imide ring-closing ratio of 80% or more (see Patent Literature 1). In this case, by simply volatilizing a solvent from the varnish, an anchor coat layer containing the polyimide can be formed, and this can lower the heating temperature.